Dewaxing processes employing constrained intermediate pore molecular sieves as catalysts possess greater selectivity than conventional catalytic dewaxing processes. To improve catalytic activity and to mitigate catalyst aging, these high selectivity catalysts often contain a hydrogenation/dehydrogenation component, frequently a noble metal. Such selectivity benefit is derived from the isomerization capability of the catalyst from its metallic substituent and its highly shape-selective pore structure. ZSM-23, and some other highly selective catalysts used for lube dewaxing, have a unidimensional pore structure. This type of pore structure is particularly susceptible to blockage by coke formation inside the pores and by adsorption of polar species at the pore mouth. Therefore, such catalysts have been used commercially only for dewaxing "clean" feedstocks such as hydrocrackates and severely hydrotreated solvent extracted raffinates. In the development of shape selective dewaxing processes, key issues to be addressed are retardation of aging, preservation of high selectivity over the duration of the catalyst cycle, and maintenance of robustness for dewaxing a variety of feedstocks.
U.S. Pat. No. 4,222,543 (Pelrine) and U.S. Pat. No. 4,814, 543 (Chen et al.) were the earliest patents to disclose and claim the use of constrained intermediate pore molecular sieves for lube dewaxing. U.S. Pat. No. 4,283,271 (Garwood et al.) and U.S. Pat. No. 4,283,272 (Garwood et al.) later claimed the use of these catalysts for dewaxing hydrocrackates in energy efficient configurations. Also directed to dewaxing with constrained intermediate pore molecular sieves are U.S. Pat. No. 5,135,638 (Miller), U.S. Pat. No. 5,246,566 (Miller) and U.S. Pat. No. 5,282,958 (Santilli). None of these patents was, however, directed to catalyst durability. Pelrine's examples were directed to start-of-cycle performance with furfural raffinates as feeds. The catalysts used in Pelrine's examples typically age rapidly when exposed to these feeds.
Previous inventions have addressed the problem of catalyst aging and extension of cycle length in dewaxing processes involving intermediate pore zeolites, such as ZSM-5. The techniques disclosed in these inventions are not generally applicable to the catalysts of this invention. U.S. Pat. No. 5,456,820 (Forbus et al.) discloses a process in which a lube boiling range feedstock is catalytically dewaxed in the presence of hydrogen over a catalyst comprising an intermediate pore zeolite in the decationized form. Catalyst cycle length was found to be improved by optimizing the sequencing of various solvent extracted feedstocks.
U.S. Pat. No. 4,892,646 (Venkat et al.) discloses a process for increasing the original cycle length, subsequent cycle lengths and the useful life of a dewaxing catalyst comprising an intermediate pore zeolite (i.e., ZSM-5) and preferably, a noble metal such as Pt. The catalyst is pretreated with a low molecular weight aromatic hydrocarbon at a temperature greater than 800.degree. F., for a time sufficient to deposit between 2 and 30% of coke, by weight, on the catalyst. The pretreatment may be conducted in the presence of hydrogen gas.
U.S. Pat. No. 4,347,121 (Mayer et al., hereinafter Mayer) claimed catalytic dewaxing of hydrocrackates containing less than 10 ppm nitrogen with a hydrofinishing step upstream of the dewaxing catalyst. Mayer is, however, directed to ZSM-5 and ZSM-11. The hydrofinishing step is employed for the purpose of base oil stabilization not to improve the aging characteristics of ZSM-5 or ZSM-11. Commercial experience dewaxing hydrocrackates with ZSM-5 shows negligible aging.
Chen, et al (U.S. Pat. No. 4,749,467), discloses a method for extending dewaxing catalyst cycle length by employing the combination of low space velocity and a high acidity intermediate pore zeolite. The high acid activity and low space velocity reduce the start-of-cycle temperature. Because catalyst deactivation reactions are more temperature sensitive than are dewaxing reactions, low operating temperatures reduce the catalyst aging rate. The same principle has been found to apply to unidimensional constrained intermediate pore molecular sieves.
Dewaxing catalysts comprising intermediate pore molecular sieves containing noble metals have been found to have relatively high aging rates when dewaxing heavy hydrocrackate feeds at a space velocity of 1 LHSV or greater. The catalyst eventually lines out at high temperature, resulting in non-selective cracking and significant yield loss. The aging rate and yield loss with time can be reduced somewhat by operation at a relatively low space velocity. Additionally, noble metal-containing constrained intermediate pore catalysts age very rapidly when exposed to feedstocks having even modest levels of nitrogen and sulfur, such as mildly hydrotreated solvent refined feeds or hydrocrackates produced at low hydrocracker severity.
It has been discovered, however, that the use of a high activity hydrotreating catalyst (a catalyst which can operate effectively at high space velocities and relatively low temperatures is considered a high activity catalyst) upstream of the dewaxing catalyst (preferably in one vessel, creating a synergistic catalyst system) is extremely effective for reducing the dewaxing catalyst aging rate and eventual line out temperature. The synergistic catalyst system also permits operation at significantly higher space velocities than would be possible with the dewaxing catalyst operating alone. The synergistic combination of hydrotreating and dewaxing catalysts offers the potential for longer cycle length while processing difficult feeds with moderate amounts of nitrogen, sulfur and aromatics, such as low conversion hydrocrackates. This invention is also effective with hydrotreated raffinates and some neat raffinates. This is an unexpected improvement, since nitrogen and sulfur are generally known to be effective poisons for catalysts loaded with noble metals.
There are also economic advantages from the invention. It is significantly less expensive to load a dewaxing reactor with a combination of hydrotreating catalyst and noble metal containing dewaxing catalyst than it is to load a reactor with the dewaxing catalyst alone. This also avoids gas separation and clean-up typical of prior art.
The prior art discussed in the background above demonstrates that previous attempts to retard aging and yield loss have been focused on restricting conditions of the dewaxing process to specific parameters, such as temperature or space velocity. Alternately, the dewaxing catalyst itself has been altered by additional steps such as precoking or is formulated to high alpha requirements, both of which can reduce catalyst selectivity. The instant invention retards aging much more effectively than methods previously disclosed. It is also much less expensive and time consuming to implement.
The dewaxing catalysts of this invention are very effective hydrogenation catalysts when acting alone, nearly completely saturating the aromatics in the feed. It is, therefore, unexpected that adding a high activity hydrotreating catalyst ahead of, and preferably in, the same reactor with the dewaxing catalyst results in dramatic minimization of aging. Catalyst line-out time and eventual equilibration temperature are reduced. Furthermore, the upper space velocity limit for stable operation of the dewaxing catalyst is substantially extended. The catalyst combination of the instant invention appears to have a different aging mechanism than the dewaxing catalyst operating alone, permitting higher space velocity operation simultaneously with a lower aging rate.
The synergistic catalyst combination of the instant invention performs well for hydrocracked feeds in addition to permitting the processing of feeds with moderately high levels of nitrogen and sulfur. Such feeds would ordinarily poison either of these catalysts alone causing rapid and uncontrollable aging.
The invention may be summarized as follows:
A process for catalytically dewaxing a lubricant feedstock whereby the aging of the dewaxing catalyst and eventual line-out temperature are minimized. Applicable feedstocks are preferentially hydrocrackates or hydrotreated raffinates but include raffinate products of conventional solvent extraction processes. The feedstock is contacted in the presence of hydrogen with the catalyst system at a space velocity (based on the dewaxing catalyst volume) between 0.2 and 10 and in a temperature range between 450.degree. F. and 800.degree. F. The catalyst system comprises a high activity hydrotreating catalyst operating upstream of a dewaxing catalyst, preferably (although not restricted to operating) in the same reactor vessel. The hydrotreating and dewaxing catalysts each preferably contain one or more noble metals with the dewaxing catalyst also containing a constrained intermediate pore molecular sieve.